1. Technical Field
The present invention relates to providing attenuation in a Vector Network Analyzer (VNA) receiver to prevent non-linear receiver signal output when measuring insertion loss or gain of a device under test (DUT).
2. Related Art
Currently available two port VNAs attenuate the reference channel as much as practical to overcome the leakage signal from the reference channel to the test channel. The resulting constructive and destructive addition of the desired measurement signal through the DUT causes a ripple signal to appear. A limitation is reached when the reference signal is decreased enough to cause noticeable noise when measuring low insertion loss devices due to the excess noise in the reference channel itself. This forces the two (or more) measurement channels of the VNA to be designed with inherently high isolation at a considerable cost.
Due to these high noise isolation requirements, the design of a combined source and detector, such as a VNA, used to measure the insertion loss or gain of a DUT will typically be bounded by a number of constraints discussed to follow.
As a first constraint, the maximum input to the receiver is limited by the point at which the receiver starts to exhibit nonlinearity. For a microwave sampler this level is about −21 dBm. For mixers, depending on the LO drive level, nonlinearity can be exhibited at 0 dBm.
As another constraint, the minimum input to the receiver is first limited by KTB Noise. The KTB noise is fixed at −174 dBm in a 1 Hz Bandwidth receiver at 25 Degrees Celsius. This noise increases by a factor of 10 Log(Bandwidth) in Hz. The minimum input to the receiver is also limited by the Noise Figure (NF) of the receive chain. Sampler based systems have a NF of around 30 dB. Mixer based systems have a NF of around 10 dB. The relative merits of mixer vs. sampler based systems is primarily due to complexity of the mixer, although LO frequency generation is also more complex in mixers. Wide bandwidth mixers are constrained by the internal Balun structures, which limit the mixer operation to 100s of KHz to 100s of MHz, or 100s of MHz to 20 to 40 GHz.
Isolation is another constraint on receivers. Isolation between mixers is dependent on their inherent RF to LO isolation plus the isolation between the two (or more) LO drive circuits. Isolation between the two LO drive circuits is usually determined by a splitter. The splitter is also constrained by its internal Balun structure. The splitter is limited to the same frequency ranges as the mixers. Samplers on the other hand are simple devices in comparison. The LO (Sampler turn on drive) is a narrow pulse on the order of 10s of picoseconds. The isolation between samplers is accomplished with simple orthogonal microstrip to coplanar waveguide transitions. The LO pulse frequency only has to cover one octave, starting at the lowest frequency of operation. Higher frequencies are converted using under sampling. For convenience, further description will be limited to sampler based systems, although techniques according to embodiments of the present invention are understood to work equally well with mixer based systems. A typical planar dual sampler driven with a common pulse will exhibit about 50 dB of isolation between sampler inputs. Laboratory grade samplers can reach 85 dB. These devices rely on very complex splitters. They are physically large due to the 3D construction of the pulse coupling through the orthogonal plane. Further examples herein are limited to splitters with a limit of 50 dB of isolation. These constraints define the upper and lower limits of a receiver.
To measure loss and gain using a source/receiver of a conventional VNA, attenuation is typically provided to the reference channel enough to keep the leakage signal >10 dB below the KTB Noise Floor of the Test Channel. The drawback to this technique is that the KTB noise of the reference channel dominates in low loss measurements.
FIG. 1 shows components of a VNA connected to measure a DUT to provide a reference to describe how noise is limited. In FIG. 1, the gain or attenuation of each component is labeled. The labels are used to analyze the Reference (R) signal component and the Transmitted (T) signal component as measured by the VNA.
The components illustrated in FIG. 1 for a VNA are conventional and include the signal source 2 for providing a test signal to coupler 4. The test signal is provided on a through path of the coupler 4 and experiences an incident signal loss (IL) before being provided through a device under test (DUT) 8. The test signal from the DUT 8 is then received through attenuator (A2) 10 and provided for down conversion in downconverter 14. The downconverter 14 generates harmonics, creating the NFT, or noise figure component for the test signal. The bandpass filter 18 passes the desired harmonic of the test signal (T) to the VNA receiver for evaluation. The receiver noise of the test system 22 is illustrated to be added in at summer 24.
The signal from source 2 is also provided through the coupling path of coupler 4 as a reference signal, and experiences a coupling loss (CPL) before reaching the attenuator (A1) 6. The reference signal from the attenuator is provided to downconverter 12. The downconverter 12 generates harmonics, creating the NFR, or noise figure component for the reference signal. The bandpass filter 16 passes the desired harmonic of the reference signal (R) to the VNA receiver for evaluation. The receiver noise of the test system 20 is illustrated to be added in at summer 26.
A synopsis of each component and its attenuation contribution is labeled as follows:
PIN: VNA Signal Source (2)
IL: Insertion Loss of Through Path of Coupler (4)
CPL: Loss through Coupling Path of Coupler (4)
A1: Attenuation through attenuator A1 (6)
DUT: Attenuation through DUT (8)
A2: Attenuation through attenuator A2 (10)
NFR: Noise Figure of reference downconverter (12)
NFT: Noise Figure of transmitted downconverter (14)
ISO: Isolation Attenuation (15) between downconverters (12) and (14)
BWR: Bandwidth of the Reference Filter (16) in KHz
BWT: Bandwidth of Transmitted Filter (18) in KHz
NSR: Noise source contribution of reference receiver (20)
NST: Noise source contribution of transmitted receiver (22)
A number of equations identified below are used in conventional systems to determine the values of A1 and A2, as well as noise in the reference (R) and test (T) signals. The values for A1 and A2 in the equations are determined assuming NFR=29 dB, and BWR=23 KHz. The equations to determine A1 and A2 are as follows:R=PIN−CPL−A1NSR=10*Log BWR+NFR−174NSR=101.4 dBm (For NSR=29 dB and BWR=23 KHz)R/NSR=R−NSRT=PIN−IL−DUT−A2NST=10*Log BWT+NFT−174NST=101.4 dBm (For NFT=29 dB and BWT=23 KHz)T/NST=T−NSTR=R−ISOT/RI=T−RI=T−R+ISOLet T/R1=T/NST+Delta (T/R1>T/NST by Delta dB)T=R+ISO=T−NST+DeltaT−(PIN−CPL−A1)+ISO=T−NST+DeltaNST−Delta=PIN−CPL−A1−ISOA1=PIN−CPL−NST−ISO+Delta  (Equation 1 for A1)T=PIN−IL−DUT−A2A2=PIN−IL−DUT−T  (Equation 1 for A2)
Next, to provide values for A1 and A2 in a typical system, as an example it is assumed that T=−21 dbM at DUT=0 dB. Also, PIN is +9 dB, IL is −4 dB, CPL is −14 dB, ISO is −50 dB and Delta is 15 dB. Applying these values, the following values for A1 and A2 are determined from Equation 1 for A1 and Equation 1 for A2 as follows:A2=PIN−IL+21  (From Equation 1 for A2)A2=9−4+21A2=26 dBA1=PIN−CPL−NST−ISO+Delta  (From Equation 1 for A1)A1=9−14−(−101.4)−50+15A1+61.4 dB
Next formulas for the signal to noise ratio are determined for both the reference signal (R) relative to NSR and the test signal (T) relative to NST, and the test signal (T) relative to R1. The formulas are derived as follows:T/NST=T−NSTT/NST=PIN−IL−DUT−A2−NSTT/NST=+9−4−DUT−26−(101.4)T/NST=80.4−DUTT/R1=T−R+ISOT/R1=PIN−IL−DUT−A2−R−ISOT/R1=PIN−IL−DUT−A2−(PIN−CPL−A1)+ISOT/R1=PIN−IL−DUT−A2−PIN+CPL+A1+ISOT/R1=CPL+ISO+A1−IL−A2−DUTT/R1=95.4−DUTR/NSR=R−NSRR/NSR=PIN−CPL−A1−NSRR/NSR=+9−14−61.4−(101.4)R/NSR=35.5 dB
For a given signal to noise ratio in dB (SN), the converted Noise Signal (NS) in dB is:NSdB=20*((Log(1+10−(SN/20)−Log(1−10−(SN/20))
For a given signal to interfering signal ratio (SIS) in dB, the converted Ripple (RIP) in dB is:RIPdB=20*(Log(1+10−(SIS/20))−Log(1−10−(SIS/20))
For two uncorrelated S/N Noise Sources VN1 and VN2 in dB, the total Noise VNT in dB is:VN1=10(SN1/20) VN2=10(SN2/20) VNT=SQR((VN2)+(VN2)2))NSTdB=20*((Log(1+VNT)−Log(1−VNT))
FIG. 2 shows a plot of S/N ratio in dB for three different measured signals versus DUT loss in dB. First, a plot 200 of the reference channel signal to noise ratio R/NSR is provided vs. DUT loss. Second, a plot 202 of the test channel signal to noise ratio T/NST is provided vs. DUT loss. Third a plot 204 of the test channel signal to interfering signal T/R1 is provided vs. DUT loss.
FIG. 3 shows a plot of converted noise signals and ripple versus DUT loss in dB. First, a plot 302 of the converted reference channel S/N Rns is provided vs. DUT loss. Second, a plot 300 of the converted test channel S/N ratio Tns is provided vs. DUT loss. Third, a plot 304 of the total converted noise Tns+Rns is provided. Fourth, a plot of the ripple 306 is provided.
As seen from FIG. 3, the ripple stays 15 dB (Delta) below the test channel noise, thus making it transparent. It is also apparent that the total noise due to Rns is dominant for DUT=<45.4 dB. This is the unfortunate consequence of A1 being dependant on isolation to get the ripple below the noise.